Mellon optical memory

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Mellon optical memory was an early form of computer memory invented at the Mellon Institute in the 1950s. The device used a clever combination of photoemissive and phosphorescent materials to produce a "light loop" between two surfaces. The presence or lack of light, detected by a photocell, represented a one or zero. Although promising, the system was rendered obsolete with the introduction of core memory in the early 1950s. It appears that the system was never used in production, but it represents one of the typically odd earlier attempts to produce a useful high-speed memory system.

Contents

[edit] Construction

A Mellon memory device consisted of a grid of individual "cells", each holding one bit, sandwiched between a glass backing and held in vacuum. Behind one of the glass layers was the photoemissive material, which was charged to a high negative DC voltage. When light struck the layer a shower of electrons was released, amplified by the external power. The electrons were pulled towards the positive potential, placed on the phosphorescent layer. When the electrons struck this material, light was generated. The light filled the interior of the cell, striking the photoemissive layer, which continued to emit electrons. This process would continue for a short time; the light emitted by the phosphorescent layer was much smaller than the amount of energy absorbed by it from the electrons, so the total amount of light in the cell faded away at a rate determined by the characteristics of the phosphorescent material.

[edit] Writing

Writing to the cell was accomplished by an external cathode ray tube (CRT) arranged in front of the photoemissive side of the grid. Cells were selected by using the deflection coils in the CRT to pull the beam into position in front of the cell, lighting up the front of the tube in that location. This initial pulse of light, focussed through a lens, would set the cell to the "on" state. Due to the way the photoemissive layer worked, focusing light on it again when it was already "lit up" would overload the material, stopping electrons from flowing out the other side into the interior of the cell. When the external light was then removed, the cell was dark.

[edit] Reading

Reading the cells was accomplished by a grid of photocells arranged behind the phosphorescent layer, which emitted photons from both sides as long as it was thin enough. To form a complete memory the system was arranged to be regenerative, with the output of the photocells being amplified and sent back into the CRT to refresh the cells periodically.

[edit] Williams tube

Overall the system was very similar to the more well-known Williams tube. The Williams tube used the phosphorescent front of a single CRT to create small spots of static electricity on a plate arranged in front of the tube. However, the stability of these dots proved difficult to maintain. The Mellon system replaced the static charges with light, which was much more resistant to external influence.

[edit] References

  • Echert Jr., J.P., A Survey of Digital Computer Memory Systems, Proceedings of the IRE, October 1953. Reprinted in 'IEEE Annals of the History of Computing, Vol.20, No.4, 1998